Geochemistry of chromite from middle-deep VYZHIGA MASSIF (SOUTH-EATS FENNOSCANDIA)

V.V.Kulikova¹, V.S.Kulikov¹, Ya.V.Bychkova²,

Institute of Geology, Karelian research centre RAS , Petrozavodsk, vkulikova@onego.ru

² Geological faculty, MSU, Moscow, yanab66@yandex.ru

Vyzhiga layered massif is situated in the central part of South-East Fenoskandia between Burakovsko-Aganozersky intrusion and paleorift Vetreny Poyas. Generalized section of the intrusion 500 m thick includes different zones (from the bottom to the top): Metapyroxenitic zone  (chlorite-tremolitic schistic rocks) – about 5 m; Peridotitic zone with unit thin layeres of olivinitic chromitite and a horizone of websterite – about 200 m; Pyroxenitic zone – about 100 m; Gabbronoritic zone – more than 20 m; Laikogabbroic zone (dioritic) – about 150 m.

The results of 59th drill (“Arkhangelskgeologiya company) detail research of the west part of Vyzhiga massif are the subject of these abstracts. This drill strips the bottom part of the intrusion from the contact with the containing granitoids.

The following intervals can be marked in the drill section (in meters from the top up to the bottom):

0 – 25.0. morainic deposits and the crust of weathering of quaternary mafic-ultramafic rocks.

25.0 – 92.4. serpentinous poikilitic plagioclase-content lherzolite (MgO – 35-40 mas %, chemical analyses are recalculated to anhydrous totals of 100%) with clinopyroxene oikocrysts  (about 3х2 sm). At the depth of 82.45 - 82.75 m a layer of olivine chromitite was diagnosed.

92.4 – 103.0. fine-grained websterite (MgO – 25 – 26 %).

103.0 – 106.0. plagioclase-content olivine websterite (MgO – 26 – 30 %).

106.0 – 153.0. serpentinous poikilitic plagioclase-content lherzolite (MgO – 32 – 35 %). At the depth of 120.0-122.3  m a layer of olivine chromitite was diagnosed. At the depth of 127.5 m. there are small lenses of Ni, Cu and Ag sulfides.

153.0 – 170.0. serpentinous poikilitic plagioclase-content lherzolite (MgO – 35-40%).

170.0 – 190.0. serpentinous lherzolite лерцолиты (MgO – 26 – 32 %). At the depth of 178.3 – 179.0 m a layer of olivine chromitite was diagnosed.

190.0 – 197.0. chlorite-tremolite rock (metomorphosed pyroxenite)  (MgO – 24 – 26 %).

197.0 – 200.0. kataclased containing granitoid.

Authors primary research chrome-spinel (about 200 granes) by microprobe zond Vega@Tescan1 (Institute of Geology, Karelian SC V.Kulikova). Two groups of chrome-spinel were separated: rock-building (I) and и accessory (II) (fig. 1).

I group was is situated in the olivine chromitite layeres. Its concentration reaches 60% of the rock volume. Chromite crystals from the 82.65 m and 122.6 m horizons (fig, 2)  are idiomorphic, unzoned, their size is  0.1 – 0.3 mm. They are characterized by the highest  Cr₂O₃  concentration (49 - 55%). At the same time chrome-spinel «82.65» have lower conentration of MgO (4.33-5.70%), than chrome-spinel «122.6» (7.86 – 10.54%), and higher concentrations of FeO (31.58 – 35.10% and 25.10 – 31.75%). In some crystals concentration of MnO are about 0.77%, V₂O₅  - 0.77%, Dy₂O₃ - 2.91%, NiO -  6.62%.

II group is represented by accessory chrome-spinel and can be segregate into three sub-groups. It is caused by different composition of containing rocks: lherzolite, websterite and metamorphosed lherzolite. Variable concentration of MgO (1.75 – 5.72%), Al₂O₃ (5.45 – 11.42%), TiO₂ (0.63 – 5.47%), Cr₂O₃ (31.29 – 50.79%) and FeO (35.38 – 59.60%) is typical for accessory chromite from lherzolite. Some concentration of MnO, V₂O₅, Dy₂O₃, NiO is in their composition. Chrome-spinel from websterite are characterized by the ZnO concentration of 0.83%. Concentration of FeO is high (71.64 – 79.37%) and of Cr₂O₃ is low (18.94 – 24.73 %) in the chrome-spinel from the metalherzolite.

Chrome-spinel composition from the “main chromite horizone” of Burakovso-Aganozersky massif is the similar to this from 59 drill in Vyzhiga (depth of 122.6 m). Due to Lavrov data [1] there is a lens of fine-grained clinopyroxene in olivine chromitite cumulate. Its size is 2-6 sm. Chrome-spinel has the following  composition: high ccjmcentration of MgO and Al₂O₃ (about 8 - 11%), TiO₂ - no more than 1.5% and Cr₂O₃ - 53– 5.5%.

At the ATM(Al₂O₃/TiO₂) – MgO [2] diagram every group and sub-group form their individual field. Accessory chrome-spinel composition from the Vetreny Poyas komatiitic basalt were not researched detail, but thanks to some data and new data from Myandukha volcano (we propose, that Undozersky intrusion was its undervolcano chamber) we can segregate them into the individual group. It has gradation, which correlate with the sample spatial position in the paleorift.

Chrome-spinel are discovered over the depth of 172 m. Upper the composition changes to chromite, such minerals as magnetite and sulfides (even argentite) occur. We propose it is the result of endocontact entertainment during crystallization and the late metasomatose.

For the first time “critical horizone” was discovered at the depth of 193.7 m. It is situated 3.5 m high from the inner contact with the containing granitoid. “Critical horizone” is represented by drop-like magnetite globs; it stretches toward drill 58 about 2 km. They form shelly multilayer aggregates gathered together into clusters. Mineral composition in the drill 58 is: TiO₂ - 1.02, V₂O₅ - 0.97, FeO - 98.01; in the drill 59 (193.7 m depth) their are not V₂O₅, TiO₂ concentration is not more than 0.44%. Here their are a lot of solid solution disintegration structures (magnetite-titanomagnetite-ilmenite system). Single grains of platinoid and Au are discovered.

At the first time for Vyzhiga massif virgin Ag, Fe in serpentine and some other new minerals were discovered. The most significant is unusual composition of zircon, a lot of REE minerals (monazite, xenotime, apatite), variable composition of zircon and baddeleyite.

New geological and mineralogical data for Vyzhiga massif , which is the important link in the chain of  intrusions from different depth of Vetreny Poyas volcano-plutonic association, are perspective to unique research of petrogenesis and oregenesis evolution.

New data can testify both to genetic unity Vyzhiga intrusion and Burakovsko-Aganozersky massif and to multistage mafic-ultramafic melt intrusion. The stages were: Burakovskaya → Shalozerskaya → Burakovskaya → Vyzhiga ± Lumbasruchei volcano (?) → Kamennaya Gora ± Karbasnikova volcano (With Victoria lava lake), Ruiga ± Goletz volkcano. We suggest that new data will help to argue the conception of paleoproterozoic magmatic mafic-ultramafic system forming at the base of mantle plume Windibelt existence and successive degradation during this time.

1.                   Lavrov M..M., Trophimov N.N. New chromite province of Karelia // OIM in 1984. Petrozavodsk. 1988. pp. 39-42.

2.                   Petrochemical series of magmatic rocks ... //V.V.Kulikova et al. Petrozavodsk. 2001. 115 p.